Air-Intake-Filter-Screen Technology Helps Optimize AHU Performance

May 01, 2008

At a major university in the Midwest, one of the main air-handling units (AHUs) serving a 65,737-sq-ft biomedical building was experiencing ongoing problems during winter: On one side of the building, snow drawn through the intake louver would accumulate on MERV 8 pre-filters until its weight and moisture caused the filters to collapse, exposing expensive MERV 15 bag filters relied upon to provide fine filtration.

FILTER MAINTENANCE

The maintenance team had to change 30 pre-filters 12 times a year: eight times (every two weeks) during winter and four times during spring, summer, and fall. Each of the 12 changes took 2.5 man-hours and cost approximately $215 in materials and labor (a total of $2,580). Thirty bag filters were changed twice a year, with each change taking four man-hours and costing $1,095 in materials and labor (a total of $2,190). Excluding inspection time — during winter, the filters had to be checked constantly — the approximate annual cost of maintaining the filters was $4,770.

With no elevator in the section of the building where the AHU is located, maintaining the 30 pre-filters involved two trips (the pre-filters come 24 to a box) up and down several flights of stairs. Maintaining the 30 bag filters involved 15 trips (two large filters per box) up and down the stairs.

SOLUTION

In 2006, the maintenance team learned of air-intake-filter-screen technology designed and engineered by Air Solution Co. for use on high-volume/high-velocity HVAC and cooling-tower systems.

Designed to have little impact on airflow and static pressure, the screens stop airborne debris at its point of entry. At the university, they were mounted inside of the air chamber, behind the intake louvers, where they are accessible from inside the building, rather than mounted to the outside of the intake louvers, which are located several stories up and, thus, are inaccessible.

The screens not only have eliminated snow-related damage during winter, they have stopped cottonwood seed and other matter from prematurely fouling the pre-filters during spring, summer, and fall. (The biomedical building is located near a river, and the campus is surrounded by densely wooded areas.)

“It wasn't until we saw the cottonwood seed building up on the air-intake filter screens that we realized just how serious our cottonwood-seed problem was,” the facility maintenance engineer said. “The cottonwood seed and other fibrous matter in the air was increasing our maintenance cost and significantly reducing the efficiency and service life of our filters.”

The screens take about 10 min to clean using a broom.

Since the screens were installed, pre-filter changes have been reduced from 12 to three a year, while bag-filter changes have been reduced from two to one a year. When nearby construction is completed, the university expects to be able to reduce bag-filter changes to one every other year.

With the screens, the university is saving $1,935 a year on pre-filter changes and $1,095 a year on bag-filter changes, for a combined annual savings of $3,030.

Reducing the number of filter changes is consistent with the university's campuswide green-building initiative, as it contributes to the university's aggregate waste reduction. As a result, the maintenance team is working to implement air-intake-filter-screen technology on other equipment across campus.

COST JUSTIFICATION

The total cost of the air-intake filter screens was $7,885. With an expected service life of 15 years, the screens have an annualized cost of nearly $525 ($7,855 ÷ 15 years). Add to that the annual cost of three pre-filter changes ($645) and one bag-filter change ($1,095), and the total annual cost of the air-intake filter screens is $2,265. Compared with the previous annual filter-maintenance cost of $4,770, that equates to a savings of $2,505 and a payback of just over 3.1 years.